Tuning the Charge of Sliding Water Drops

When a water drop slides over a hydrophobic surface, it usually acquires a positive charge and deposits the negative countercharge on the surface. Although the electrification of solid surfaces induced after contact with a liquid is intensively studied, the actual mechanisms of charge separation, so-termed slide electrification, are still unclear. Here, slide electrification is studied by measuring the charge of a series of water drops sliding down inclined glass plates. The glass was coated with hydrophobic (hydrocarbon/fluorocarbon) and amine-terminated silanes. On hydrophobic surfaces, drops charge positively while the surfaces charge negatively. Hydrophobic surfaces coated with a mono-amine (3-aminopropyltriethyoxysilane) lead to negatively charged drops and positively charged surfaces. When coated with a multiamine (N-(3-trimethoxysilylpropyl)diethylenetriamine), a gradual transition from positively to negatively charged drops is observed. We attribute this tunable drop charging to surface-directed ion transfer. Some of the protons accepted by the amine-functionalized surfaces (−NH2 with H+ acceptor) remain on the surface even after drop departure. These findings demonstrate the facile tunability of surface-controlled slide electrification.


Supplementary Experimental Section Atomic Force Microscopy
Scanning force microscopy (SFM) has been used to map the topography of surfaces. We performed the SFM studies with a soft tapping mode in air (Bruker, Dimension ICON). As the probe, and OTESPA silicon probe is used (Bruker, nominal spring constant 26 N/m and a nominal resonance frequency of 300 kHz).

Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)
TOF-SIMS analysis was performed on pristine and drop-wetted surfaces using an IONTOF TOF.SIMS 5 NCS (Münster, Germany). The analyzer was operated at a cycle time of 150 μs corresponding to a mass range of 1-2070 u. 30 keV Bi 3 + cluster ions were used as primary ions (current: 0.10 pA) rastering a field-of-view of 200 x 200 μm². A 2.5 keV Ar 1000 gas cluster ion beam (current: 0.18 nA) was employed for depth profiling sputtering an area of 400 x 400 µm².
The SurfaceLab software (Iontof, Münster, Germany) version 7.1 was used to evaluate the data.
In TOF-SIMS, surfaces are bombarded with primary ions, resulting in the emission of secondary ions sputtered from the surface material. These ions are analyzed and the obtained mass spectra can be used to identify the composition of the material. The analysis of spectra from the pure chemical variants allows the identification of species which are indicative of the analyzed material (diagnostic ions). During depth profiling, one cycle of analysis is followed by a surface erosion step using a higher current ion gun. Repeating these alternating steps, a depth profile revealing the course of the ion signals with increasing depth is obtained. The diagnostic ions identified are fragments of the organic chains, which allow the identification of the presence of the materials in the layer system.

Surface Chemical Compositions: TOF-SIMS
To provide an improved understanding behind the chemical configuration of surfaces For PFOTS-APTES, the C 3 Ntrace initially decayed at a lower speed compared to CF 3 -.
Thereafter, both ions, CF 3and C 3 N -, decay at similar rates ( Figure S2b, insets). For PFOTS-NTDET, a comparatively fast initial decay of C 3 Noccurs, in fact, similar to the decay of CF 3 -.
Thereafter, the diagnostic ion C 3 Ndecayed slower, at a similar gradient to APTES's C 3 N -( Figure S2c, insets). The decay profiles may indicate that amines in PFOTS-APTES are more evenly distributed throughout the entire depth as compared to PFOTS-NTDET. throughout the depth of the surfaces (purple data points, Figure S2 insets). In addition, the presence of proton acceptors (amines) is also prevalent in the sub-surface depths (blue data points, Figure S2 insets). As wetting is primarily influenced by the first 1-3 nm while electrification is deeper (3-5 nm), 1 this configuration is optimal for preserving hydrophobic wettability while integrating charging characteristics. Therefore, the primary layers of PFOTS and TCPS inhibit wetting and drop fragmentation while the secondary amine layers provide the desired slide electrification.     and drop length). a) The mobility changes described in the main manuscript occurs significantly despite fairly minor alterations to the measured advancing and receding contact angles. However, the method used (tilting plate) may be simply incapable of detecting the change of receding contact angles at a sub-10 µm scale. b) The drop length for PFOTS-NTDET during sliding changes considerably more than that for PFOTS. Together, changes in drop profile are likely caused by surface adaptation, attributed to surface chemistry and wetting alterations. These combined factors lead to changes in drop profile, hence adhesion, and mobility.